This type of planar microlens array was disclosed by Hamanaka, et al. in Japanese Patent Publication No. 3-136004 (Refer to FIG. 1). The planar microlens array disclosed in the Publication is manufactured using the ion diffusion process. In the ion diffusion process, a glass substrate is used as a transparent substrate 1 on the surface of which a diffusion-inhibiting mask film comprising Ti, Al, Ni or Cr, etc. is formed using the sputtering and other thin-film forming processes, and then a desired array of circular mask apertures is formed on the diffusion-inhibiting mask film. After that, the transparent substrate is immersed for a predetermined period of time in a molten-salt bath containing ions contributing to increasing the refractive index of the substrate. As ions in the molten-salt bath are diffused through the mask apertures in the transparent substrate, a planar microlens array 82 having a plurality of microlenses of a refractive-index distribution type with decreasing refractive index from the vicinity of the center of each mask aperture toward the circumference thereof is formed. In a liquid crystal display panel having a delta (six-lobe) pixel array, this planar microlens array has microlenses of a hexagonal circumferential shape that are arranged two-dimensionally, regularly and densely on the surface of the transparent substrate, as shown in FIG. 1. In this case, the lens array is a six-lobe array.
The circumferential shape of each microlens can be made into a desired hexagonal shape by providing a region 6 where the diffusion fronts 5 that are leading edges of the microlens ion diffused areas of the adjoining microlenses 4 are fused together, as shown in FIG. 2. In this case, lens filling rate, the ratio of the area occupied by microlenses to the total area of the transparent substrate on which lenses are formed, is almost 100%.
Oikawa et al., on the other hand, disclosed a planar microlens array for use in liquid crystal display panels in Japanese Patent Publication No. 5-45642 (Refer to FIG. 3). A planar microlens array 82 is formed with the ion diffusion process using a diffusion-inhibiting mask film having formed oblong mask apertures. This planar microlens array 82 has microlenses 4 of an oblong circumferential shape, as shown in FIG. 4, by terminating ion diffusion in the state where diffusion fronts of the adjoining microlenses come in contact with each other. With this, the lens filling rate is short of 100% because of the existence of a lens unformed region where no lenses are formed, but a planar microlens array 82 having a relatively high lens filling rate is formed since the lens array is of a six-lobe type.
These planar microlens arrays are suitable for condenser lenses to illuminate display images by converging light to illuminate a transmissive display panel, such as a liquid crystal display panel having a plurality of pixels, onto pixel apertures. A typical projection-type image display device having such a planar microlens array is shown in FIG. 5.
In FIG. 5, the light emitted by a light source 86 is converged by a reflecting mirror 87 and a condenser lens 85 so as to direct toward a projection lens 89. The light flux passing through the condenser lens 85 is converged by a planar microlens array 82 onto a pixel aperture area (pixel aperture) 81b of a liquid crystal element formed between two substrates 81a of a liquid crystal panel 81, and projected onto a screen 88 by the projection lens 89.
By using a planar microlens array as described above, the light flux that would otherwise been shielded by the pixel shielding area 81c of the liquid crystal display panel 81 can be converged onto the pixel aperture area 81b. As a result, the light flux reaching the screen 88 increases 2 to 2.5 times as much as that without the planar microlens array 82.
The technologies disclosed in Japanese Patent Publication No. 3-136004 and Japanese Patent Publication No. 5-45642 are intended to improve overall converging efficiency in converging light onto the pixel apertures of the liquid crystal element. To further improve converging efficiency, it is generally necessary to further increase the lens filling rate of the planar microlens array. With the six-lobe lens array as disclosed in Japanese Patent Publication No. 3-136004, the lens filling rate is almost 100%, with virtually no room left for further improvement. In terms of the improvement of overall converging efficiency, however, there still remains a problem. That is, regions where the diffusion fronts of the adjoining lenses are fused together contribute little to the conversion of light onto the pixel apertures that is illuminated because of their high astigmatism due to lowered rotational symmetry of the concentration distribution of diffused ions. In other words, an increase in the area where diffusion fronts are fused together could lead to a decrease, far from an improvement, in overall converging efficiency, even with 100% of the lens filling rate.
Now, a model for forming two microlenses on a flat substrate with an ion diffusion process using circular mask apertures will be taken up as an example for simplicity. FIGS. 6A through 6C are partially cross-sectional perspective views illustrating the state where two microlenses are formed on a transparent substrate 1. Ions are diffused through a circular mask aperture as a diffusion center 52, and diffusion fronts 5 that are leading edges of diffusion areas spread in concentrical semispheres. With the progress of ion diffusion, the two diffusion fronts eventually come into contact with each other (FIG. 6A). As ion diffusion further proceeds, the diffusion fronts are fused together, and the ion concentration gradient in the direction of a line connecting the adjoining diffusion centers decreases in the fused area. Thus, the diffusion speed in that direction decreases, and as a result, the diffusion fronts in the fused area form a continuum of curved surface, expanding in the direction orthogonally intersecting the aforementioned direction (in the direction of the boundary line with the adjoining lens) (FIGS. 6B and 6C). FIG. 7 is a perspective view illustrating a solid body formed by the diffusion fronts viewed from the direction opposite to the surface on which lenses are formed. The area in which these diffusion fronts have been fused together is called an overlapped diffusion area 6. The length of the overlapped diffusion area 6 in the direction of a line connecting the lens centers is called the width W of the overlapped diffusion area 6.
As the overlapped diffusion area 6 receives ions fed by both the diffusion centers 52 as the ion sources, the concentration distribution of diffused ions forms a gentle saddle shape in the direction of a line connecting the lens centers (in the X direction shown in FIG. 7). The overlapped diffusion area 6 has a lower refracting power than an equivalent independent microlens, causing a remarkable astigmatism, because of the smaller gradient of its ion concentration distribution in the X direction. Consequently, the light incident on this overlapped diffusion area 6 hardly converge on the neighborhood of the focal point of each lens on which the light is originally to converge, but on a narrow strip-like area connecting the centers of the adjoining lenses. The model of this state is shown in FIG. 8. In the figure, numeral 41 denotes a focal point of the microlens 4, 42 a converging area of the microlens, and 43 a converging plane, respectively. The light incident on the overlapped diffusion area 6 also converges on a strip connecting the focal points 41.